The present invention relates to the rotating blades of a gas turbine. More specifically, the present invention relates to a scheme for cooling the platform portion of a gas turbine blade.
A gas turbine is typically comprised of a compressor section that produces compressed air. Fuel is then mixed with and burned in a portion of this compressed air in one or more combustors, thereby producing a hot compressed gas. The hot compressed gas is then expanded in a turbine section to produce rotating shaft power.
The turbine section typically employs a plurality of alternating rows of stationary vanes and rotating blades. Each of the rotating blades has an airfoil portion and a root portion by which it is affixed to a rotor. The root portion includes a platform from which the airfoil portion extends.
Since the vanes and blades are exposed to the hot gas discharging from the combustors, cooling these components is of the utmost importance. Traditionally, cooling is accomplished by extracting a portion of the compressed air from the compressor, which may or may not then be cooled, and directing it to the turbine section, thereby bypassing the combustors. After introduction into the turbine, the cooling air flows through radial passages formed in the airfoil portions of the vanes and blades. Typically, a number of small axial passages are formed inside the vane and blade airfoils that connect with one or more of the radial passages so that cooling air is directed over the surfaces of the airfoils, such as the leading and trailing edges or the suction and pressure surfaces. After the cooling air exits the vane or blade it enters and mixes with the hot gas flowing through the turbine section.
Although the approach to blade cooling discussed above provides adequate cooling for the airfoil portions of the blades, traditionally, no cooling air was specifically designated for use in cooling the blade root platforms, the upper surfaces of which are exposed to the flow of hot gas from the combustors. Although a portion of the cooling air discharged from the upstream vanes flowed over the upper surfaces of the blade root platforms, so as to provide a measure of film cooling, experience has shown that this film cooling is insufficient to adequately cool the platforms. As a result, oxidation and cracking can occur in the platforms.
One possible solution is to increase the film cooling by increasing the amount of cooling air discharged from the upstream vanes. However, although such cooling air enters the hot gas flowing through the turbine section, little useful work is obtained from the cooling air since it was not subject to heat up in the combustion section. Thus, to achieve high efficiency, it is crucial that the use of cooling air be kept to a minimum.
It is therefore desirable to provide a scheme for cooling the platform portions of the rotating blades in a gas turbine using a minimum of cooling air.